Emit data definition example values as the JSON Schema "examples" keyword

Problem/Motivation

#3620922 lets data definitions declare example values through getExamples() and setExamples(). The values are descriptive metadata. The json_schema serialization format does not read them, so generated schemas omit the standard examples keyword.

Steps to reproduce

N/A

Proposed resolution

Extend the annotation helper added in #3623431. PrimitiveDataNormalizer and TypedDataNormalizer emit the examples keyword. These normalizers output the value in the format setValue() accepts, so the annotation matches the declared schema type.

• Examples which cannot be represented in JSON are dropped individually. The keyword is emitted when at least one example remains.
• \Stringable examples are cast to strings. A NULL example is kept.
• JSON:API's FieldNormalizer removes item-level examples from the array schema of a multi-value field.

Remaining tasks

Review. Postponed on #3620922 and #3623431.

User interface changes

None

API changes

SchematicNormalizerHelperTrait's protected casting method is renamed to serve both keywords. No other changes to existing methods.

Data model changes

None

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